How to perform insect classification given two images of the same insect?
There are several ways you can do this. 

One is to input both images in input, so it can be a 2 input system or an input with 6 channels.

As you suggested in 1st point, you can make 2 networks, connect them at the end and add another layer for final classification or use outputs from both and train another classifier (like Gradient bosting). You can look up ensemble techniques and ensemble of neural networks for more.

For dual classification and naive approach, you will again have a problem in case there are some insects that can solely be identified from features at a specific angle, if so you can then discard other angles (e.g. ignore moth face if moth back is the distinguishing feature)